Fuel thieves in Mexico drilled 10,363 illegal taps into state-owned pipelines in 2017, or an average of about 28 illegal taps every day.

The government-owned oil company Pemex reported Wednesday that illegal taps rose by about 50 percent, compared to the 6,873 found in 2016.

Those are just the taps that have been found; many others are believed to be still working surreptitiously, drawing off gasoline or diesel that is sold illegally or through established gas stations.

Last year, the government estimated the 2016 thefts cost Pemex about $1 billion a year.

The scale of such thefts has grown exponentially; just five years ago, Mexico found only about 1,635 illegal taps annually.

The thefts are carried out by drug gangs, and heavily armed criminal gangs.

In the northern border state of Tamaulipas, for example, the army reported Wednesday that soldiers had found several stashes of stolen fuel, drugs and weapons.

The raids Tuesday and Wednesday near the border city Reynosa turned up about 5,500 gallons (21,000 liters) of presumably stolen fuel, cocaine, 17 assault rifles, two .50-caliber sniper rifles and two claymore mines.

Three people were detained in the raids.

The fuel thefts were once largely confined to two or three states in Mexico, but have since spread across the country, including the suburbs of Mexico City.

In Tamaulipas, once the leader in such thefts, 1,100 illegal taps were found in 2017. But 1,852 such taps were found in the north-central state of Guanajuato, 1,443 in the state of Puebla, 1,064 in Hidalgo and 1,012 in Veracruz.

In the State of Mexico, which borders Mexico City, 975 taps were found.

Thieves use hand drills to make a hole in pipelines and quickly screw valves and hoses into place; they then fill everything from 275-gallon (1,064 liter) portable tanks to entire tanker trucks.

The illegal taps are usually discovered either because they leak, make pipeline pressure drop or catch fire.